Exam questions · Biology · Culturing Microorganisms and Developing Medicines
Investigating Antiseptics, Antibiotics and Plant Extracts
- 6 exam questions
- 19 marks
- 9 quick checks
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1 State [1 mark]
State what a clear zone around a disc shows.
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Model answer
The substance on the disc stopped bacteria growing, or killed them.
Mark scheme
- Bacteria were killed or did not grow — 1 mark
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2 Explain [2 marks]
A disc soaked in sterile water was included in the investigation. Explain why.
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Model answer
It is a control. If there is no clear zone around it, this shows that the water and the paper disc do not stop the growth of bacteria, so any zones round the other discs are due to the substances being tested.
Mark scheme
- A control for comparison — 1 mark
- Shows the zones are due to the substances tested — 1 mark
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3 Calculate [3 marks]
The clear zone around an antiseptic disc had a diameter of 20 mm. Calculate the cross-sectional area of the zone. Give your answer in mm squared to 3 significant figures.
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Model answer
Radius = 10 mm. Area = π × 10² = 314 mm².
Mark scheme
- Radius of 10 mm — 1 mark
- Use of π r² — 1 mark
- 314 mm squared — 1 mark
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4 Calculate [3 marks]
A student found that substance X gave a zone of diameter 18 mm and substance Y a zone of diameter 9 mm. Calculate how many times larger the area of the zone for X is than that of Y.
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Model answer
Area X = π × 9² = 254 mm². Area Y = π × 4.5² = 63.6 mm². 254 ÷ 63.6 = 4 times larger.
Mark scheme
- Area of X — 1 mark
- Area of Y — 1 mark
- 4 times — 1 mark
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5 Describe [4 marks]
Describe four precautions that should be taken to make this investigation safe.
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Model answer
Use harmless bacteria and not known pathogens. Use aseptic technique, so that other microorganisms are not grown. Tape the lid and do not open the dish after incubation. Wash hands, wipe the bench with disinfectant, and autoclave the dishes before disposal.
Mark scheme
- Harmless bacteria — 1 mark
- Aseptic technique — 1 mark
- Taped lid and not opened — 1 mark
- Hygiene, or autoclave for disposal — 1 mark
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6 Evaluate [6 marks]
A student tested an antibiotic, an antiseptic and a garlic extract on a lawn of bacteria. The areas of the clear zones were antibiotic 452 mm squared, antiseptic 314 mm squared and garlic 78 mm squared, and there was no zone around the control. Evaluate what the results show about the three substances, and suggest how the reliability of the investigation could be improved.
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Model answer
The antibiotic made the biggest zone, which shows that it was the most effective at stopping the bacteria from growing, followed by the antiseptic and then the garlic. The control had no zone, so the zones were caused by the substances. However, a bigger zone does not mean that a substance is safe or better as a medicine, because the antiseptic could harm human cells. The garlic extract works, but is weaker than the others, and it is not known what it contains, or whether it would be safe to take. Only one test was done, so the results might be a chance event. Repeating at least three times and calculating a mean area would make the results more reliable, and using the same concentration for each substance would make the comparison fairer.
Mark scheme
- Antibiotic biggest zone, most effective — 1 mark
- Order of effectiveness — 1 mark
- Control shows the zones are due to the substances — 1 mark
- Limit: antiseptic can harm human cells, or garlic is not known to be safe — 1 mark
- Repeat the investigation and find a mean — 1 mark
- Keep the concentration and the other variables the same — 1 mark
Quick check
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1
What is a zone of inhibition?
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B: A clear area around a disc where bacteria did not grow
It is a clear area where bacteria did not grow.
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2
Why is a sterile water disc used?
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C: As a control
It is a control that shows the zones are due to the substances.
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3
A clear zone has a radius of 5 mm. What is its area, to the nearest whole number?
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D: 79
\(\pi \times 5^2 = 78.5\), which is 79 mm squared.
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4
A clear zone has a diameter of 10 mm. Which calculation gives its area?
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A: \(\pi \times 5^2\)
The radius is 5 mm, so the area is \(\pi \times 5^2\).
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5
If the diameter of a clear zone doubles, what happens to its area?
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C: It becomes four times as large
Area depends on the radius squared, so it is four times as big.
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6
Why are the dishes not opened after incubation?
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B: To stop bacteria being released
The bacteria could be harmful, and would be released.
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7
Which is a control variable in this investigation?
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A: The temperature of incubation
The temperature of incubation must be the same for all of the discs.
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8
Why are antiseptics not used as medicines inside the body?
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D: They can harm human cells
They can damage human cells as well as microorganisms.
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9
A plant extract produces a large zone. Why can it not yet be called a medicine?
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B: It has not yet been tested for safety and effectiveness in humans
It must also be shown to be safe and effective in people, through further testing.